1. Field of the Invention
The present invention relates to an aqueous ink particularly suitable for use in an ink jet recording method, and an ink set, an image forming method and an image forming apparatus using such an aqueous ink.
2. Description of the Related Art
An ink jet recording method is widely used in various fields because of the features that recording can be conducted with a low noise, and high-resolution images can be obtained at high speed by using a recording head having a high-density multi-orifice.
Inks (pigment inks) containing a pigment as a coloring material capable of obtaining recorded articles high in image density and excellent in fastness properties have heretofore been used as inks used in the ink jet recording method. With respect to the pigment inks, inks containing a dispersant such as a polymer for the purpose of stably dispersing a pigment in an ink, improving scratch resistance and highlighter resistance of images and forming a smooth film on a recording medium have been proposed (see Japanese Patent Application Laid-Open No. H08-239610).
On the other hand, the pigment inks involve a problem that an image density is lowered when an image is formed by applying the pigment ink and a color ink so as to overlap each other. To solve such a problem, proposals have been made on a method of preventing an image formed with the pigment ink from lowering the image density thereof upon the formation of the image by applying the pigment ink and the color ink so as to overlap each other (see Japanese Patent Application Laid-Open Nos. 2001-150793, 2001-152059, 2002-154263 and 2004-115549). Specifically, proposals have been made on enhancing the image density by using the pigment ink and a color ink containing a component for aggregating a pigment in the pigment ink.
The present inventors have carried out an investigation on such prior art that the image is formed by applying the pigment ink containing the dispersant and the color ink containing the component for aggregating the pigment so as to overlap each other as described above. As a result, it has been found that when a recording medium, plain paper in particular, is used, the image density of the image formed with the pigment ink is not sufficiently achieved according to the kind of the recording medium. It has also been found that the lowering of the image density is markedly caused when the pigment ink is applied so as to overlap the color ink in particular.
The present inventors infer the reason why the image density is not sufficiently achieved as follows. More specifically, the dispersion state of the pigment in the pigment ink containing the dispersant is relatively stable, and so the tendency of the pigment to aggregate is relatively low. Therefore, the color ink to be used together with such a pigment ink fails to allow an aggregated pigment to be sufficiently present on a recording medium where the color ink contains only the component for aggregating the pigment. It is thus considered that the image density is not sufficiently achieved because a part of the pigment, which has not been aggregated at this time, permeates into the interior (thickness-wise direction of recording medium) of the recording medium.
The present inventors have carried out an investigation on such a problem. As a result, it has been found that the lowering of the image density caused by applying the pigment ink containing the dispersant so as to overlap the color ink can be prevented to some extent by adding the component for aggregating the pigment in excess into the color ink. It has however been found that when a polyvalent metal salt generally used as the component for aggregating the pigment is added in excess into the ink, a problem may occur on the ejection stability of the ink from a recording head in some cases.
Thus, the present inventors have also carried out an investigation with a view toward preventing the lowering of the image density by using a color ink containing the polyvalent metal salt to the same extent as the content of the polyvalent metal salt in a conventional ink. As a result, it has been found that setting the surface tension (static surface tension) of the color ink higher than that of the conventional general ink can prevent the lowering of the image density. It has however been found that when the pigment ink containing the dispersant and such a color ink are applied so as to overlap each other, a problem that the fixing ability of an image formed with the pigment ink is lowered occurs because the surface tension of such a color ink is high, and so the permeability thereof into the recording medium is low.